Purpose:
Young children have reduced interpupillary distance and hyperopic refraction relative to adults, leading to decreased convergence and increased accommodation demands. Children with typical amounts of hyperopia accommodate to achieve clear retinal images, at the same time generating convergence through neural coupling. It is known that adults demonstrate vergence adaptation to a variety of visual stimuli. Henson and North (1980) reported adults showing average adaptation of 72% to a 6pd demand after 3.5 mins. Do young children also show adaptation to their immature demands?

Methods:
Purkinje image tracking and eccentric photorefraction were used to record eye alignment and accommodation in adults and young children (3.25 - 5 yrs). Participants viewed a naturalistic target at 95cm binocularly (bi) for 15 secs, followed by 15 secs of monocular (mo) viewing. Each heterophoria was derived from the difference between these two alignments. Additional vergence demand was introduced by holding a 2 MA base out prism (10pd in adults; 6pd in kids) in front of one eye for an extended period of time (~2.5-3.5 mins). The heterophoria measurements were repeated during this period. Adaptation was defined as the percentage of the additional prism that was compensated for by changes in phoria position.

Results:
90% of adults maintained fusion when viewing through prisms and on average adapted to 52% of the additional demand (range 39%- 72%). 86% of young children also fused and on average adapted to 55% of the additional demand (range 38% - 67%). When comparing accommodation during the adaptation period with baseline, accommodation was somewhat increased in the beginning of the adaptation (0.25±0.13D bi, 0.02±0.09D mo in adults; 0.28±0.18D bi, 0.25±0.18D mo in kids) and greater still at the end of adaptation (0.31±0.19D bi, 0.14±0.27D mo in adults; 0.33±0.13D bi, 0.15±0.14D mo in kids).

Conclusions:
Young children and adults showed heterophoria adaptation to a 2 MA demand at a 95cm distance. Changes in accommodation responses during the adaptation were significantly smaller than the changes in phoria. In typically developing children who experience changing visual demands, this adaptation might help maintain eye alignment. Its role in clinical abnormalities is yet to be determined.